1. Field of the Invention
The invention relates to an image forming apparatus using an intermediate transfer member contacting with an image bearing member, and particularly to an image forming apparatus which can improve the quality of a toner image transferred from an image bearing member to an intermediate transfer member.
2. Related Background Art
In an image forming apparatus using an intermediate transfer member, a toner image is formed on an image bearing member such as a photosensitive drum. Then, this toner image is once primary-transferred to an intermediate transfer member in a primary transfer portion. Thereafter, the toner image on the intermediate transfer member is secondary-transferred from the intermediate transfer member onto a recording material such as paper in a secondary transfer portion.
A resin material is widely used as a material forming the intermediate transfer member. As specific examples thereof, polyvinylidene fluoride (PVdF) is described in Japanese Patent Application Laid-open No. H5-200904, polycarbonate (PC) is described in Japanese Patent Application Laid-open No. H6-149081, and polyimide is described in Japanese Patent Application Laid-open No. S63-311263.
The resin materials are excellent in mechanical characteristic. On the other hand, however, an intermediate transfer member formed of a resin material is small in the amount of deformation when it is brought into pressure contact with an image bearing member. That is, the amount of deformation of the intermediate transfer member in a primary transfer portion is small. Thus, in the primary transfer portion, great pressure acts on a toner from the intermediate transfer member.
Further, pressure from intermediate transfer concentrates in a portion of the intermediate transfer member on which the amount of toner is locally great when the toner is transferred from the image bearing member to the intermediate transfer member.
Thereupon, the toner image on the portion in which the pressure concentrates is crushed. For example, when the toner image is a line-shaped image, there arises the problem that the line width on the image bearing member is widened by the toner image being transferred to the intermediate transfer member.
In order to solve this problem, Japanese Patent Application Laid-open No. H10-97146 proposes an intermediate transfer member provided with an elastic layer. When an elastic layer is provided on the intermediate transfer member, the hardness of the intermediate transfer member becomes smaller than the hardness of the toner. Thereupon, even if as described above, there exists a portion in which the amount of toner is locally great, the intermediate transfer member is deformed along the toner and therefore, the concentration of the pressure in the portion wherein the amount of toner is great is alleviated. Accordingly, the crushing of the toner image by the concentration of the pressure is suppressed.
However, if the intermediate transfer member having the elastic layer is used, there will arise the problem that when the toner image is transferred from the image bearing member to the intermediate transfer member, there occurs the so-called “scattering” that toner particles scatter around the toner image transferred to the intermediate transfer member.
FIGS. 7A, 7B, 7C and 7D of the accompanying drawings show the mechanism of the occurrence of the scattering.
FIG. 7A represents the toner “t” of a toner image present on an image bearing member (photosensitive drum) 1.
As shown in FIG. 7B, when the toner “t” is transferred from the image bearing member 1 to an intermediate transfer member 51, the intermediate transfer member provided with an elastic layer is deformed along the toner “t”. By the intermediate transfer member 51 being deformed, even if there exists a portion on which the amount of toner is locally great in the toner image on the photosensitive drum 1, the concentration of pressure in this portion on which the amount of toner is great is suppressed.
Here, pressure is substantially uniformly applied from the intermediate transfer member 51 to the toner image on the image bearing member 1. Thereupon, the toner image on the image bearing member 1 is transferred to the intermediate transfer member 51 while substantially maintaining its shape.
FIG. 7C is a typical view of the toner image transferred to the intermediate transfer member 51.
As shown in FIG. 7C, the toner “t” in the upper layer portion of the toner image transferred to the intermediate transfer member 51 becomes unstable. That is, as shown in FIG. 7C, the toner “t” in the upper layer portion becomes liable to move in the direction indicated by the arrow. Thereupon, as shown in FIG. 7D, the scattering occurs.